This invention relates generally to quadrupole ion trap mass spectrometers and more particularly to a quadrupole ion trap mass spectrometer having an ion volume defined by spaced linear rods in which image currents produced by ion motion in the trapping volume are detected by electrodes located between the spaced rods. The image currents are Fourier analyzed to provide an indication of the mass of the ions in the trapping volume.
Fourier transform ion cyclotron resonance mass spectrometers (FT-ICR) have demonstrated many performance capabilities that are of significant analytical value, including high resolution, high mass accuracy and multi-stage mass spectrometry. The primary disadvantage of the FT-ICRs is that they require both magnetic and electric fields. Although the previous difficulties with stray magnetic fringe fields affecting vacuum pumps, electronics and computers, have apparently been solved through the use of actively or passively shielded magnets, this solution is provided at an additional cost over standard superconducting magnets. Furthermore, the size, cost and complications associated with cryogenics produce an instrument that is large, expensive, and difficult to maintain.
The quadrupole ion trap is currently a commercial success because of its sensitivity, low cost and benchtop design. However, the ion trap is being challenged, in terms of performance, by a new generation of benchtop time-of-flight instruments equipped with orthogonal acceleration. Although incapable of MS/MS, these instruments provide high scan rates (10-100 scans/sec), medium resolution (5000-7000 FWHM), and pseudo-accurate mass capabilities when combined with internal standards (2-10 ppm). Improvements are possible to the quadrupole ion trap to bring the mass accuracy to time-of-flight levels, but these results cannot be obtained on a time scale required for chromatographic investigations.
Implementation of a Fourier transform mode of operation on the quadrupole ion trap provides an instrument with the low cost and moderate size, but with the high resolution and mass accuracy of the FT-ICR. Based on the current ion trap sizes (7-10 mm) and RF electronics (17kV p-p), frequency dispersions equivalent to very high magnetic fields ( greater than 10T) can be produced with a small benchtop instrument. The large frequency dispersion can be exploited to increase either the scan rate or the resolving power of the ion trap.
The major obstacle to operating the quadrupole ion trap in Fourier transform mode is the need to detect uV-level image signals from the trapped ions in the presence of feedback from the kV trapping field. The first demonstration of operating a quadrupole ion trap in FT mode was by Syka (U.S. Pat. No. 4,755,670: Fourier Transform Quadrupole Mass Spectrometer and Method). It was demonstrated that with appropriate filtering and nulling of the RF signal, ion signals could be detected over a narrow mass range. However, the detection sensitivity was relatively poor, and the ion densities required for obtaining useful signal caused significant ion coupling.
Frankevich et al., U.S. Pat. No. 5,625,186 describe an ion trap mass spectrometer of the type having an ion trapping volume defined by spaced end caps and a ring electrode. The ion trap includes a small sensing electrode, which senses characteristic motion of ions trapped in said trapping volume and provides an image current. Ions are excited into characteristic motion by application of an excitation pulse to the trapped ions. The pin picks up a fraction of the ion image current, while the end cap shields the pin detector from the RF and reduces capacitive pickup. The elimination of the filtering and nulling circuitry improves the detection sensitivity significantly so that reduced ion densities can be used to separate ions and enable detection of signals over a broader mass range. This original system has recently been improved by the use of differential detection, which reduces background noise while also increasing signal and simultaneously minimizing the effects of ghost-peaks created by non-sinusoidal image current pickup.
Another approach to reduce the coupling of the RF trapping field to the detection circuit is described by Aliman and Glasmachers xe2x80x9cA Novel Electric Ion Resonance Cell Designed with High Signal-to-Noise Ratio and Low Distortion for Fourier Transform Mass Spectrometryxe2x80x9d (J. Am. Soc. Mass Spectrom. (1999) 10, 1000-1007). By using a segmented cylindrical ring electrode and capacitive shimming, the feedback from the ring electrode was significantly reduced. Initial results using single-ended detection are similar in signal-to-noise and resolving power as that demonstrated by the Frankevich et al.
Although further improvements are expected with these instruments, they have yet to produce analytically useful signals at ion densities which eliminate ion coupling.
It is an object of the present invention to provide a Fourier transform linear quadrupole ion trap having an electrode geometry which minimizes the trapping field feedback.
It is another object of the present invention to provide a linear quadrupole ion trap having a large useful ion trapping volume.
It is a further object of the present invention to provide an ion trap for a Fourier transform mass spectrometer which has low field distortion.
There is provided an ion trap mass spectrometer of the type which includes a trapping volume defined by a quadrupole structure including spaced linear quadrupole rods in which ions can be trapped by application of RF and DC voltages to the rods. Independent image current electrodes are disposed between said quadrupole rods to detect image currents in response to movement of ions trapped in said trapping volume by application of an excitation voltage which causes the ions to move towards and away from said image current electrodes to generate image currents.